Control method and device for I-type three-level circuit

1. A control method for an I-type three-level circuit, characterized in comprising: detecting a current of an inductor connected with each of high-frequency arm bridges in an operation state in the I-type three-level circuit; and in a positive half cycle of an alternating-current connection terminal voltage of the I-type three-level circuit, when a freewheeling switching tube of the high-frequency arm bridge connected with the inductor is in ON state and a main switching tube of the high-frequency arm bridge connected with the inductor is in OFF state, controlling the freewheeling switching tube in ON state to keep in ON state and the main switching tube in OFF state to keep in OFF state if it is detected that the current of the inductor does not reach a preset negative current, and controlling the freewheeling switching tube in ON state to be turned off and the main switching tube in OFF state to be turned on if it is detected that the current of the inductor reaches the preset negative current, wherein the I-type three-level circuit includes bus capacitors Co+ and Co−, when the alternating-current connection terminal voltage is in the positive half cycle, the bus capacitor Co+ operates while the bus capacitor Co− remains idle, and when the alternating-current connection terminal voltage is in a negative half cycle, the bus capacitor Co− operates while the bus capacitor Co+ remains idle.

2. The control method of claim 1 , characterized in that prior to controlling the freewheeling switching tube in ON state to keep in ON state and the main switching tube in OFF state to keep in OFF state, further comprising: determining that an instantaneous value of the alternating-current connection terminal voltage of the I-type three-level circuit is greater than or equal to a half of a direct-current connection terminal voltage.

3. A control method for an I-type three-level circuit, characterized in comprising: detecting a current of an inductor connected with each of high-frequency arm bridges in an operation state in the I-type three-level circuit; and in a negative half cycle of an alternating-current connection terminal voltage of the I-type three-level circuit, when a freewheeling switching tube of the high-frequency arm bridge connected with the inductor is in ON state and a main switching tube of the high-frequency arm bridge connected with the inductor is in OFF state, controlling the freewheeling switching tube in ON state to keep in ON state and the main switching tube in OFF state to keep in OFF state if it is detected that the current of the inductor does not reach a preset positive current, and controlling the freewheeling switching tube in ON state to be turned off and the main switching tube in OFF state to be turned on if it is detected that the current of the inductor reaches the preset positive current, wherein the I-type three-level circuit includes bus capacitors Co+ and Co−, when the alternating-current connection terminal voltage is in a positive half cycle, the bus capacitor Co+ operates while the bus capacitor Co− remains idle, and when the alternating-current connection terminal voltage is in the negative half cycle, the bus capacitor Co− operates while the bus capacitor Co+ remains idle.

4. The control method of claim 3 , characterized in that prior to controlling the freewheeling switching tube in ON state to keep in ON state and the main switching tube in OFF state to keep in OFF state, further comprising: determining that an instantaneous value of the alternating-current connection terminal voltage of the I-type three-level circuit is greater than or equal to a half of a direct-current connection terminal voltage.

5. A control device for an I-type three-level circuit, characterized in comprising: a detecting unit configured to detect a current of an inductor connected with each of high-frequency arm bridges in an operation state in the I-type three-level circuit; and a control unit configured to, in a positive half cycle of an alternating-current connection terminal voltage of the I-type three-level circuit, when a freewheeling switching tube of the high-frequency arm bridge connected with the inductor is in ON state and a main switching tube of the high-frequency arm bridge connected with the inductor is in OFF state, control the freewheeling switching tube in ON state to keep in ON state and the main switching tube in OFF state to keep in OFF state if it is detected that the current of the inductor does not reach a preset negative current, and control the freewheeling switching tube in ON state to be turned off and the main switching tube in OFF state to be turned on if it is detected that the current of the inductor reaches the preset negative current, wherein the I-type three-level circuit includes bus capacitors Co+ and Co−, when the alternating-current connection terminal voltage is in the positive half cycle, the bus capacitor Co+ operates while the bus capacitor Co− remains idle, and when the alternating-current connection terminal voltage is in a negative half cycle, the bus capacitor Co− operates while the bus capacitor Co+ remains idle.

6. The control device of claim 5 , characterized in that the control unit is further configured to, prior to controlling the freewheeling switching tube in ON state to keep in ON state and the main switching tube in OFF state to keep in OFF state, determine that an instantaneous value of the alternating-current connection terminal voltage of the I-type three-level circuit is greater than or equal to a half of a direct-current connection terminal voltage.

7. A control device for an I-type three-level circuit, characterized in comprising: a detecting unit configured to detect a current of an inductor connected with each of high-frequency arm bridges in an operation state in the I-type three-level circuit; and a control unit configured to, in a negative half cycle of an alternating-current connection terminal voltage of the I-type three-level circuit, when a freewheeling switching tube of the high-frequency arm bridge connected with the inductor is in ON state and a main switching tube of the high-frequency arm bridge connected with the inductor is in OFF state, control the freewheeling switching tube in ON state to keep in ON state and the main switching tube in OFF state to keep in OFF state if it is detected that the current of the inductor does not reach a preset positive current, and control the freewheeling switching tube in ON state to be turned off and the main switching tube in OFF state to be turned on if it is detected that the current of the inductor reaches the preset positive current, wherein the I-type three-level circuit includes bus capacitors Co+ and Co−, when the alternating-current connection terminal voltage is in a positive half cycle, the bus capacitor Co+ operates while the bus capacitor Co− remains idle, and when the alternating-current connection terminal voltage is in the negative half cycle, the bus capacitor Co− operates while the bus capacitor Co+ remains idle.

8. The control device of claim 7 , characterized in that the control unit is further configured to, prior to controlling the freewheeling switching tube in ON state to keep in ON state and the main switching tube in OFF state to keep in OFF state, determine that an instantaneous value of the alternating-current connection terminal voltage of the I-type three-level circuit is greater than or equal to a half of a direct-current connection terminal voltage.